Metabolism and sensitivity to drugs and chemical carcinogens differ among tissues, organs, individuals and species. The type and quantity of specific forms of cytochrome P-450 determine the extent of activation and/or detoxification of particular substrates. Monoclonal antibodies (MAbs) are specific probes for cytochrome P-450 isozymes. The MAbs to rat cytochrome P-450 which cross-react with human cytochromes P-450 are useful tools for identification of particular isozymes. MAbs to pregnenolone 16-Alpha-carbonitrile- and ethanol-inducible rat cytochromes P-450 detect cytochrome P-450 in human liver. MAbs to human cytochromes P-450 from placenta, mitochondria and liver were prepared. These MAbs may be useful in phenotyping human cytochrome P-450 in different tissues and organs of individuals exposed to different environments. The roles of different forms of cytochrome P-450 in the metabolism of chemical carcinogens can then be evaluated.